I've got a 86'115. I'm getting 100-110psi in 3 cylinders and only 70psi in the 4th. I was getting 60 in the low cylinder but managed to get it up with a small squirt of pb blaster and 20 minutes of run time with a premixfuel mixed with a shocker dose of ring free. Am I on the right track? Is there something else I could do?
The reason I got into all this is because the last time I was out on te lake she kept cutting out at around 3000rpm. I'm thinking my injection pump is not working right and she's starving for oil @ the higher rpm range and it's over heating.
The plugs all looked the same, somewhat clean and little tan colored (good). This is actually different from what it ad been lookinglike before all this. They used to look a little oil rich.
My hunch here is something changed during thatlast ride. I.E. the oil injector pump is starting to fail.
Once I get through the ompression issue, I want to do a flow test. The book says 2.7cc@1500rpm per line.
Is there any words of wisdom out there for all this?

Thanks for the input Joe. I'm really hoping that I can pop that ring out and get the compression to within 10psi of the others. The fact that I've got any improvement at all leaves me optimistic that it's not a broken ring. I just hope that it didn't overheat stuck.
What do you mena by exhaust resonance problems?
I will change the water pump before I take it out again. Is there a chance that some part of the cooling jacket is clogged? If so how is the best dealt with? How would I isolate where the clog could be?

Toag,
I would try a shock treatment with Seafoam to see if that might loosen up the rings in that bad cylinder:
3/4 gallon of fuel
3 oz of TCW-3 oil
16 oz can of Seafoam
Mix in small container - disconnect line from tank to primer bulb - connect short piece of fuel line to primer bulb and insert into small container. (I do this on the driveway once a year to de-carbonize my C115). Hook up ear muffs and be prepaired to smoke up the neighborhood - best to do this when wife is away shopping or something.
Run the motor at fast idle for fif**** minutes and then let set for fif**** minutes. Run again, and let set again for same time. When run out of mixture, hook up primer bulb back to main tank and run for ten minutes to clear the concentration of Seafoam out of the fuel system. Now clean up all of that nasty black carbon off the driveway before the wife gets back home.
Now add another can of Seafoam (16 oz) to approx 16 gallons of fuel and go run the old girl (the boat, not your wife) on the water for about an hour. If this does not bring up the compression on that bad cylinder, then a tear down is probably in order.
If it has been at least three years since last water pump repair, it would not hurt to do one. I would also pull the thermostats and clean and test per the Yamaha shop manual. Ditto for the PRV.
Good luck,
Ken K

Ken,
What is seafoam? I will look into changing the thermostats. I will also install a new water pump. I will install the whole thing not just the impeller. I'vebeen using PBbalster to loosen up tha ring. As of today, I've got 92 psi so far. It seems to be steady improving. I let the pb blaster soak 24 hours. i fire her up with 50:1 mix and ringfree in a shocker dose for about 10 minutes. Ive done this 3 times now. With each cycle I've improved it by 10+psi.

Toag,
Stick with what you are doing - it seems to be working. What is PB Blaster and how are you using it? Are you just squirting it into the cylinder thru the spark plug hole? What is your mixture ratio on the "shocker dose" with RingFree?
Seafoam is a strong detergent similiar to RingFree. Some outboard owners say it is more effective than RingFree, which is nothing more than Chevron Techron with Yamahas name on it.
Seafoam can be purchased at most major auto supply stores.
If I were you, I would test the thermostats before replacing them - might save you a few bucks.
Let us know how you come out.
Good luck,
Ken K

Ken,
pb blaster is a penitrating oil like wd40 only a LOT stronger! It's commonly used to loosen rusted nuts and bolts. It has a very high flash point so it won't burn for a while in direct heat. You gotta watch it though. It can make seals swell. Don't spray to much in the spark plug hole. You don't want it running down into your main bearing seal. The maint mechanics I work with over at freudenburgNOK use it all the time for stuff like this. Just watch it near rubber.